Production of porous materials by the expansion of polymer gels

ABSTRACT

A method produces porous materials by expansion of polymer gels. The porous materials can be a micro- or nano-porous polymer materials.

The invention relates to a method for the production of porous materialsby the expansion of polymer gels and to the porous materials produced bysuch a method and to a moulded body.

BACKGROUND OF THE INVENTION

The preparation of nanoporous materials is a high-ranking goal ofcurrent research. The goal is to develop methods which on the one handallow a cost-effective and straightforward access to such materials andon the other hand include the possibility of implementing said method onan industrial scale.

There are several methods for preparing nanoporous materials which,however, have not been implemented on a larger scale due to the complexpreparation and costly use of materials and machines. Hence, forexample, silicon-based aerogels having open-cell structures between 1and 30 nm could be synthesised according to Kistler by a sol-gelprocess. Due to a subsequent supercritical drying, this initially verysimple sol-gel process becomes a complicated and tedious endeavour.Here, the solvent must be extracted carefully from a prepared silica gelby supercritical drying without damaging the extremely finenanostructure (S. S. Kistler. Nature, 127:1 (1931)).

Based on Colton's and Suh's idea to saturate polymers with foamingagents and subsequently expand them above their glass temperature,Krause et al. were able to add supercritical CO₂ to polyether imides andpolyether sulfones and thus prepare nanoporous materials they calledpolymer nanofoams (J. S. Colton; N. P. Suh. Polymer Engineering andScience, 27:485-492 (1987); J. S. Colton; N. P. Suh. Polymer Engineeringand Science, 27:493-499 (1987); J. S. Colton; N. P. Suh. PolymerEngineering and Science, 27:500-503 (1987); D. F. Baldwin et al. PolymerEngineering and Science, 36:1437-1445 (1996); D. F. Baldwin et al.Polymer Engineering and Science, 36:1446-1453 (1996)). Here, thepreparation includes the saturation of a polymer film having a thicknessof several millimetres at pressures of approx. 50 bar and temperaturesfrom 100-250° C. for several hours (B. Krause et al. Macromolecules,35:1738-1745 (2002); B. Krause et al. Macromolecules, 34:8792-8801(2001)).

Merlet et al. developed self-foaming polymer systems and achieved porediameters in the range of 10 to 700 nm. Using the polymerpolyphenylquinoxaline that contains heat-labile tert-butyloxycarbonylgroups, they were able to release CO₂ and isobutene in the polymer byincreasing the temperature and thus obtain a nanoporous material (S.Merlet et al. Macromolecules, 41:4205-4215 (2008)).

JP S58-67 423 A describes the manufacture of a polycarbonate foam.

CN 1318580 describes the manufacture of a polymer foam.

U.S. Pat. No. 7,838,108 B2 has already claimed nanocellular polymerfoams. This patent describes a method for manufacturing the protectedmaterial on the basis of a typical solution process of a foaming agentin a polymer. As described in the above references, this method involvesvery high pressures, temperatures and extremely long residence times ofthe polymer in the foaming agent. Thus, also this method is uneconomicaland reasonably applicable only to small sample thicknesses. Moreover, athermal conductivity from 1 to 10 mW/(m·K) is indicated for theprotected polymer material in claim 1 of U.S. Pat. No. 7,838,108 B2,which lies in a range that can only be achieved by evacuating porousmaterials to date (vacuum insulation panels).

EP 2 185 620 B1 describes a method for preparing micro- and nanoporousxerogels based on polyurea. Generally, in the preparation of a xerogelor also an aerogel, a gel consisting of a solid and a solvent isprepared from a sol. The gel transforms into a porous material byexchanging the solvent by a gas (air, for example). Here, it must beremembered that the gel body is already the maximum volume that can beobtained after drying the porous material. Thus, the expected density ofthe porous material only depends on the ratio of solid to solvent. Thereis therefore no volume increase by an expanding foaming agent in termsof a foaming process. In addition to the inorganic silica-based aerogelsdescribed by Kistler, the mentioned patent describes a method that alsomakes organic aerogels/xerogels accessible.

EP 1 646 687 B1 describes the manufacture of nano- or microporoussyndiotactic polystyrene. Here, a gel is prepared from a suitablesolvent and syndiotactic polystyrene and subsequently processed into aporous syndiotactic polystyrene material by super- or nearcriticaldrying with CO₂. As described in this patent, during thesuper-/nearcritical drying (residual solvent content <1 wt % (percent byweight)) the solvent is removed nearly completely and the expansion tonormal pressure is carried out during 60 to 120 minutes. The examples ofthe patent describe that after drying the polymer material has the sameshape and dimensions as the original gel. Thus, also this method doesnot provide an increase in volume in terms of an expansion of a foamingagent to produce a porous polymer material but an exchange of solvent bya gas (air). The ratio of polymer to solvent in the starting gel issignificantly responsible for the resulting density of the dried porouspolymer material. Therefore, already during the gel manufacturingprocess care is taken to keep the polymer content in proportion to thesolvent as small as possible (1-50 wt %) to obtain a target materialwith the lowest possible density.

All mentioned methods are either very time-consuming or expensive.

Hence, there is a substantial interest in simplifying the existingmethods for preparing polymer foams and therefore also in improving theobtained polymer foams, which allows to reduce productions costs and toopen up new areas of application.

SUMMARY OF THE INVENTION

Now, there has been found an improved method for the production ofpolymer nano foams that is preferably designated as “Nano-Foam byGel-Acetone Foam Formation via Expansion Locking” (NF-GAFFEL). Contraryto the existing options, NF-GAFFEL is preferably carried out without theneed for expensive chemicals and uneconomical process parameters. Thebasic principle is based on the preparation of a polymer gel that isobtained from a polymer using a plasticiser (a gelling agent, forexample).

Plasticisers within the meaning of the translation are preferablyliquids (under normal conditions) which ensure that the polymer istransferred into a gel or a gel-like state.

Accordingly, they convert the polymer to a soft state and may also bereferred to as gelling agents, for example.

Here, for example, the spaces between polymer chains are swollen by theplasticiser and preferably on a nanoscopic level a kind of bicontinuousstructure consisting of polymer strands and plasticiser is formed. Inthe next step the swollen gel is contacted with a foaming agent such asCO₂, for example, under high pressure and preferably thermostated. Whenapplying the foaming agent, the parameters of pressure and temperatureare preferably selected such that the plasticiser (acetone, for example)and the foaming agent (scCO₂, that is supercritical CO₂) exist in acompletely mixed state. Namely, this allows the foaming agent topenetrate the swollen polymer in a spontaneous, barrier-free way since ahomogeneous mixture of plasticiser and foaming agent is formed forthermodynamic reasons. Preferably, the plasticiser can be extracted fromthe polymer gel by a subsequent rapid pressure decrease, whereby theglass transition point of the polymer increases dramatically. Supportedby the cooling action of the adiabatically expanding gas, the polymerthus can solidify in the form of a nanoporous material.

Thus, the invention relates to

(1) a method for the production of a micro- and nanoporous polymermaterial, wherein

(a) the polymer starting material is swollen with a plasticiser at apreset temperature whereby it changes into a visco-elastic formable (inparticular deformable) state,

(b) subsequently the swollen polymer is contacted with a foaming agentunder high pressure, and

(c) the pressure is lowered whereby the micro- and nanoporous materialsolidifies; and

(2) a micro- or nanoporous polymer material obtainable by a methodaccording to aspect (1) of the invention.

Preferably, step (b) is always carried out after step (a).

In step (b) the swollen polymer preferably forms a macroscopicallycoherent body which is subsequently contacted with a foaming agent underhigh pressure.

According to the invention, in step (b) the plasticiser and the foamingagent preferably mix in the nanometre-sized spaces of the polymerstrands.

Due to the nanostructure of the foam, properties such as low thermalconduction, high stability and optical transparency may be achieved.These foams may be used as insulation materials, optically transparentsheets having insulating properties, nanofilters, for sound absorption,for optically active elements with a large surface area, etc. The mostimportant viewpoint for the production of such materials using NF-GAFFELis the convertibility to a continuous and large-scale process. Forexample, the polymer gels can be extruded without any problems and attypical extrusion pressures and temperatures, for example, the foamingagent also achieves the properties which may be required for asuccessful implementation of NF-GAFFEL.

For instance, polymer nanofoams such as polymer nanofoams made of PMMA,PS and PVC, for example, can be produced without any problems by usingNF-GAFFEL. Preferably, the polymer is an at least partially crosslinkedcopolymer. Since the process includes the generation of a polymer gelpreferably made of a solid polymer and a plasticiser, various polymerscan be foamed by this method. In a preferred embodiment for a successfulimplementation of the method according to the invention, the correctchoice of the plasticiser for the preparation of the polymer gel andadditionally the prerequisite of a complete miscibility betweenplasticiser and foaming agent under high pressure are the criticalrequirements.

In the work of Colten, Suh and Krause (prior art) the polymer isdirectly saturated with the foaming agent under high pressure, which isa very slow process. Long saturation times are avoided by initiallycharging the polymer gel which can preferably be saturated with aplasticiser at atmospheric pressure and only then adding the foamingagent, since the diffusion process of the foaming agent into the polymergel is surprisingly fast. Thus, the new method preferably comprisesthree steps specific to the invention: 1. the new possibility to mixpolymers and foaming agents quickly on a nanoscopic level and 2. toexpand them to a nanoporous material and additionally 3. to carry outthe method in a continuous process.

Ideally, the polymer gels can be prepared at room temperature and normalpressure and this preparation is therefore an easy to implement processboth for small samples and larger quantities. Moreover, gel productioncan be accelerated by increasing the polymer surface, increasing thetemperature and by convection. For a continuous production of nanofoamsby the method of the invention (NF-GAFFEL), the polymer gels canpreferably be prepared in mixing units or by extrusion just before theactual foaming process and initially charged. Hence, this prerequisiteensures a production of nanofoams starting from a solid polymer in acontinuous process.

The inventive initially charging of the polymers in the form of a gelstructure ensures a homogeneous and time-efficient filling of thepolymers with a pressurised foaming agent. Preferably, the pressure andthe temperature should be selected such that the plasticiser and thefoaming agent form a one-phase mixture, which is the prerequisite forthe fast diffusion process of the foaming agent into the polymer gels.The gel infiltrated with foaming agent and plasticiser becomes ananocellular foam by expansion. Here, the escaping foaming agentpreferably extracts the plasticiser and results on the one hand in adramatic increase of the glass transition point of the polymer and onthe other hand in a sudden cooling which are the reasons for aspontaneous fixation of the polymer matrix.

Preferably, the polymer starting material in step (a) of the methodaccording to the invention is at least partially crosslinked.Preferably, the crosslinker content in the polymer starting materialaccording to the invention is in a range from 0.01 to 10 mol %,particularly preferably from 0.1 to 1 mol %, based on the monomer used.Preferably, in step (a) of the method according to the invention theswelling is carried out at ambient pressure, i.e., in the range of 0.8to 1.2 bar. During the swelling in step (a) the temperature ispreferably ambient temperature, i.e. in a range from 15 to 30° C. Forreasons of process technology it can be advantageous to increase thetemperature and the pressure to accelerate the swelling process.

In another embodiment the object of the invention is achieved by amicro- or nanoporous polymer material which is preferably produced bythe method according to the invention.

The mean pore size (arithmetic mean of at least 300 counted pores) ofthe micro- or nanoporous polymer material according to the invention ispreferably in a range from 0.01 to 10 μm, particularly preferably in arange from 0.05 to 0.5 μm. The mean pore size can be determined byelectron photomicrographs and counting according to example 6.

Alternatively, it may also be preferred that the maximum of the poresize distribution is in a range from 0.01 to 10 μm, particularlypreferably in a range from 0.05 to 0.5 μm. The pore size distributioncan be a Poisson distribution, for example.

Preferably, the micro- or nanoporous material is not soluble in its ownmonomer. This is a very important preferred property of the material.This clearly shows that polymer strands (i.e. polymer chains the polymermaterial according to the invention consists of) are preferably bondedto each other not only by physical forces but covalent chemical bondsexist. Therefore, the foam is preferably covalently (and not onlyphysically) bonded.

The polymer material and/or the starting material are preferablypartially crosslinked or crosslinked.

Preferably, the crosslinker content in the micro- or nanoporous polymermaterial of the invention is in a range from 0.01 to 10 mol %,preferably in a range from 0.1 to 1 mol %, particularly preferably in arange from 0.2 to 0.6 mol %, based on the monomers resulting in thepolymer. This means that in the case of a crosslinker concentration of0.01 mol % each 10,000th or for a crosslinker concentration of 10 mol %each 10th molecule is a crosslinker molecule according to statistics.The crosslinker molecules in turn connect two polymer strands with theresult that more covalent crosslinking points exist between polymerstrands with increasing crosslinker concentration. Since the polymercannot be completely dissolved without decomposition, a chemicalcharacterisation of the polymer with regard to the degree ofpolymerisation or the molecular weight distribution is not feasible.However, the swelling degree of the partially crosslinked polymer in asuitable solvent can provide information. The higher the crosslinkercontent in the polymer, the less solvent can be absorbed by the gel. Thehigher the interaction of the polymer with the solvent, the moresignificant the differences of the swelling degree as a function of thecrosslinker concentration become. Hence, for example, benzene, tolueneor styrene as solvents allow to determine even small differences ofcrosslinker concentrations in a partially crosslinked polystyrene on thebasis of the resulting swelling degree. The crosslinker has two or morepolymerisable functional groups and can preferably be divinylbenzene(DVB), ethylene glycol dimethacrylate (EGDMA) or methylene bisacrylamide(MBAA) for polymethyl methacrylate (PMMA) or polystyrene (PS), forexample. Up to now, there was the preconception that foam cannot beprepared from (partially) crosslinked polymers. One reason might be thatpartially crosslinked polymers (copolymers) are not necessarily meltablelike thermoplastic polymers and therefore cannot be integrated in astandard foam processing. Now, the inventors surprisingly found that thefoams according to the invention can be produced particularly well from(partially) crosslinked starting materials.

The nanoporous polymer foam produced by the described method ispreferably characterised by the following properties:

The mean thickness (arithmetic mean) of the webs between the pores ispreferably in a range from 5 to 50 nm, particularly preferably in arange from 10 to 35 nm. The web thickness can be determined by electronphotomicrographs and counting according to example 6.

Preferably, the micro- or nanoporous polymer material according to theinvention is partially closed-cell. Particularly preferably, the micro-or nanoporous polymer material according to the invention is partiallyopen-cell.

The thermal conductivity of the micro- or nanoporous polymer materialfilled with air at atmospheric pressure is preferably in a range from 1to 30 mW/(m·K), particularly preferably in a range from 10 to 26mW/(m·K).

The density of the micro- or nanoporous polymer material according tothe invention is preferably in a range from 10 to 300 kg/m³,particularly preferably in a range from 30 to 200 kg/m³.

In another embodiment the object of the invention is achieved by amoulded body made of the micro- or nanoporous material according to theinvention.

The moulded body is preferably sealed. That is, the pores of the mouldedbody forming its surface are closed towards the surface of the mouldedbody.

Preferably, the moulded body is a polymer particle. This polymerparticle can be part of a polymer granulate.

The moulded body preferably has a thermal conduction or thermalconductivity in the range from 1 to 30 mW/(m·K) and particularlypreferably in a range from 10 to 26 mW/(m·K).

The novel use of polymer particles or polymer granulate instead ofpolymer plates allows to reduce all process parameters such astemperature, pressure and time due to the significantly larger surfaceand said parameters are largely independent of scaling. Hence, largequantities of the material can easily be produced.

The duration of the swelling process of the polymer particles accordingto the invention with a suitable plasticiser such as acetone, forexample, for a copolymer such as polystyrene/DVB, for example(originally from several hours to several days for a polymer plate(0.5×5×5 (cm³)), is preferably in a range from 1 to 60 minutes.

The swelling plasticiser is preferably selected from the group ofsolvents that are aprotic and/or have a dipole moment below 1.84 Debye(in μ).

The use of a polymer granulate instead of polymer plates/blocks allowsto optimise all foaming process parameters in view of an economicprocess.

If the autoclave volume is larger than the expected volume of thenanofoam, discrete nanoporous polymer particles are obtained afterexpansion, for example. If the autoclave volume is smaller than theexpected volume of the nanofoam, the polymer particles are pressedtogether when expanding and form a coherent foam body (particle foambody), for example.

The polymer particles can be both poly- and monodisperse. The meandiameter (arithmetic mean) is preferably in a range from 0.2 to 2 mm.The diameter can easily be determined optically under a lightmicroscope.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Scanning electron micrographs of purchased acrylic glass foamedby the NF-GAFFEL method. The mass ratio of acrylic glass to acetone was1:3. The sample was subjected to a CO₂ atmosphere at p=250 bar, T=55°C., t=15 min and subsequently expanded (t_(exp)≈1 s). The foam densityis 0.35±0.05 g/cm³.

FIG. 2: Scanning electron micrographs of PMMA foamed by the NF-GAFFELmethod. The PMMA gel was saturated with acetone. The samples weresubjected to a CO₂ atmosphere at p=250 bar, T=55° C., t=15 min andsubsequently expanded (t_(exp)≈1 s). The crosslinker concentration(MBAA) was ν=0.2 mol % in the left foam and ν=0.7 mol % in the rightfoam. The foam densities have values of 0.35±0.05 g/cm³ (left) and0.40±0.05 g/cm³ (right).

FIG. 3: Scanning electron micrographs of PMMA foamed by the NF-GAFFELmethod. The PMMA gel was saturated with acetone. The samples weresubjected to a CO₂ atmosphere at p=250 bar, t=15 min and subsequentlyexpanded (t_(exp)≈1 s). The crosslinker concentration (MBAA) was ν=0.7mol % in all foams. The left sample was foamed at a temperature of T=35°C., the middle sample at T=55° C. and the right sample at T=75° C. Thefoam densities have values of 0.40±0.05 g/cm³ (left), 0.40±0.05 g/cm³(middle) and 0.40±0.05 g/cm³ (right).

FIG. 4: Scanning electron micrographs of PMMA foamed by the NF-GAFFELmethod. The PMMA gel was saturated with acetone. The samples weresubjected to a CO₂ atmosphere at p=250 bar, T=55° C. and subsequentlyexpanded (t_(exp)≈1 s). The crosslinker concentration (MBAA) was ν=0.7mol % in all foams. The left sample was foamed after t=5 min, the middlesample after t=15 min and the right sample after t=60 min. The foamdensities have values of 0.45±0.05 g/cm³ (left), 0.40±0.05 g/cm³(middle) and 0.40±0.05 g/cm³ (right).

FIG. 5: Scanning electron micrographs of PMMA foamed by the NF-GAFFELmethod. The PMMA gel was saturated with acetone. The samples weresubjected to a CO₂ atmosphere at p=250 bar, T=55° C., t=15 min andsubsequently expanded (t_(exp)≈1 s). The crosslinker concentration(MBAA) was ν=0.7 mol % in the left foam and ν=1.2 mol % in the rightfoam. In both samples p=0.5 mol % of modifier (EHTG) were present. Thefoam densities have values of 0.10±0.05 g/cm³ (left) and 0.25±0.05 g/cm³(right).

FIG. 6: Scanning electron micrographs of PS foamed by the NF-GAFFELmethod. The mass ratio of PS to acetone was 1:1, resp. The samples weresubjected to a CO₂ atmosphere at p=250 bar, T=55° C., t=15 min andsubsequently expanded (t_(exp)≈1 s). The crosslinker concentration wasν=0.5 mol % in the left foam and ν=1.0 mol % in the right foam. The foamdensities have values of 0.10±0.05 g/cm³ (left) and 0.25±0.05 g/cm³(right).

FIG. 7: Scanning electron micrographs of PS foamed by the NF-GAFFELmethod. The mass ratio of PS to acetone was 1:1, resp. The samples weresubjected to a CO₂ atmosphere at p=250 bar, t=15 min and subsequentlyexpanded (t_(exp)≈1 s). The crosslinker concentration (DVB) was ν=1.0mol % in all foams. The left sample was foamed at a temperature of 35°C., the middle sample at 65° C. and the right sample at 75° C. The foamdensities have values of 0.25±0.05 g/cm³ (left), 0.25±0.05 g/cm³(middle) and 0.20±0.05 g/cm³ (right).

FIG. 8: Scanning electron micrographs of PS foamed by the NF-GAFFELmethod. The mass ratio of PS to acetone was 1:1, resp. The samples weresubjected to a CO₂ atmosphere at T=65° C., t=15 min and subsequentlyexpanded (t_(exp)≈1 s). The crosslinker concentration (DVB) was ν=1.0mol % in both foams. The left sample was subjected to a CO₂ saturationpressure of p=250 bar and the right sample to a CO₂ saturation pressureof p=150 bar. The foam densities have values of 0.25±0.05 g/cm³ (left)and 0.25±0.05 g/cm³ (right).

FIG. 9: Scanning electron micrographs of PS foamed by the NF-GAFFELmethod. The mass ratio of PS to acetone was 1:1. The sample wassubjected to a CO₂ atmosphere at p=250 bar, T=60° C., t=15 min andsubsequently expanded (t_(exp)≈20 s). The crosslinker concentration(DVB) was ν=2.0 mol %. The foam density is 0.15±0.05 g/cm³.

FIG. 10: Scanning electron micrographs of PVC foamed by the NF-GAFFELmethod. The sample saturated with acetone was subjected to a CO₂atmosphere at p=250 bar, T=70° C., t=10 min and subsequently expanded(t_(exp)≈1 s). The foam density is 0.20±0.05 g/cm³.

FIG. 11: Scanning electron micrographs of PE foamed by the NF-GAFFELmethod. The sample saturated with cyclohexane was subjected to a CO₂atmosphere at p=250 bar, T=70° C., t=15 min and subsequently expanded(t_(exp)≈1 s). The foam density is 0.35±0.05 g/cm³.

FIG. 12: Scanning electron micrograph of the nanostructure of thepolystyrene material given in example 6.

FIG. 13: Scanning electron micrograph of a nanoporous polymer materialused for the determination of pore diameters with the Datinf Measurecomputer program.

DETAILED DESCRIPTION OF THE INVENTION

In the inventive method (NF-GAFFEL method) according to aspect (1) ofthe invention, the polymer starting material consists of one or severalpolymers, preferably thermoplastic polymers, alternatively preferablycrosslinked polymers, very particularly preferably of a copolymer of thegroup of polymethyl methacrylate (PMMA/crosslinker), polystyrene(PS/crosslinker), polyvinyl chloride (PVC/crosslinker), polylactide(PL/crosslinker), polyethylene (PE/crosslinker), polypropylene(PP/crosslinker), polycarbonate (PC/crosslinker) andcellophane/crosslinker. The plasticiser is preferably selected from thegroup of ketones (such as acetone) and other polar aprotic solvents andshort chain alkanes such as butane, pentane, hexane and cyclohexane, forexample; however, acetone is particularly preferred. Here, the massratio of the polymer starting material to the plasticiser is preferablyin a range from 10:0.5 to 1:3, particularly preferably in a range from10:2 to 1:1. Preferably, the swelling time is in a range from 0.1 s to100 h, particularly preferably in a range from 1 s to 1 h.

The starting material preferably contains from 0.01 to 10 mol % ofplasticiser. The crosslinker is preferably divinylbenzene (DVB),ethylene glycol dimethacrylate (EGDMA) or methylene bisacrylamide(MBAA).

The preset temperature in step (a) and/or (b), independently of oneanother, is preferably in a range from 0 to 100° C. and is particularlypreferably in step (a) in a range from 15 to 30° C. and independentlythereof in step (b) in a range from 30 to 70° C.

It is further preferred that the swelling in step (a) results in a densepacking of polymer lattices preferably having a mean particle size inthe micrometre and nanometre ranges. It is also preferred that step (a)is carried out in an extruder.

In another preferred embodiment of the method according to theinvention, the foaming agent in step (b) is selected from CO₂ and otherfoaming agents which are completely miscible with the plasticiser underhigh pressure, in particular short chain alkanes such as methane, ethaneand propane. The gel is preferably contacted with the foaming agentunder a pressure of 10 to 300 bar, particularly preferably from 50 to200 bar.

In another preferred embodiment the pressure lowering in step (c) takesplace within a time ranging from 0.1 s to 60 s in which the polymermaterial cools and solidifies.

The obtained micro- or nanoporous polymer material preferably has adensity from 0.5% to 50% of the density of the polymer starting materialand/or a mean pore size from 0.01 to 10 μm.

The NF-GAFFEL method according to the invention could alreadysuccessfully be carried out in a batch process for copolymers such asPMMA/crosslinker, PS/crosslinker and PVC/crosslinker. The preparation ofa coherent polymer gel is important for a successful production ofnanoporous polymer materials.

Preferably, the method according to the invention is carried out as abatch method and not as a continuous method.

Preferably, the parameters of pressure and temperature are selected tobe above the binary miscibility gap of the plasticiser and particularlypreferably thermostatting is carried out for a sufficient time(depending on the sample thickness or the sample volume) so that afterexpansion foams with pore diameters in the lower micrometre or nanometrerange will always be obtained.

Especially desired properties such as, for example, low densities, smallpores, open-/closed-pored foam structures, etc. may be obtained byvarying the following parameters which can be classified into threeproduction processes:

1. Production of the polymers: In the production of the polymers, thefollowing parameters are particularly preferably important for the finalproduct:

-   -   Crosslinker, modifier and copolymer and the concentrations        thereof    -   The type of radical starter and the quantity used    -   Temperature and duration of the polymerisation reaction    -   Homogenisation during polymerisation    -   After treatment of the polymer (purification, extrusion,        annealing, etc.)

2. Production of the polymer gels: In the production of the polymergels, the following parameters are particularly preferably important forthe final product:

-   -   Plasticisers and the ratio between the solid polymer and the        plasticiser    -   The duration of exposure of the plasticiser in the polymer gel    -   The saturated polymer gel or defined plasticiser concentrations

3. Production of the polymer foams: here, the following parameters areparticularly preferably important:

-   -   The type of foaming agent    -   Pressure, temperature, time    -   Expansion speed

Preferably, the following parameters are required for the exactcomposition of the polymer gels.

The mass fraction of the radical starter used refers to the total massof the sample.

$\sigma = \frac{m_{starter}}{\sum\limits_{i}^{j}m_{i}}$

The mass fraction of the plasticiser in a plasticiser/polymer mixture isdefined as follows:λ=m _(gelling agent)/(m _(gelling agent) +m _(polymer)).

For an exact composition of the polymer, the concentrations of alladditives such as, for example, the crosslinker and the modifier aregiven on a molar basis as follows:

The molar ratio of a crosslinker in a crosslinker/monomer mixture isgiven byυ=n _(crosslinker)/(n _(crosslinker) +n _(momomer))·100.

The molar ratio of additional additives such as, for example, a modifierin an additive/monomer crosslinker mixture is given by:

$\rho = {\frac{n_{additive}}{\sum\limits_{i}^{j}n_{i}} \cdot 100.}$

The polymer gel is preferably obtained in time-economic steps dependingon the processing form. Thus, for thin films of the order of 1 μmpreferably a direct contacting with the plasticiser is expedient.Preferably, bulk materials require either polymer latex powders whichare compacted and then soaked with the plasticiser or polymer granulateswhich are obtained by various methods such as milling or freeze drying.Preferred are polymer starting materials with one dimension, length,width or height, on the order of micrometres to obtain a fast swelling.

After the provision of the required polymer gels the desired foam can beproduced by the method (NF-GAFFEL) according to the invention. Both abatch process and a continuous method are suitable for a large-scaleimplementation of the foaming process. A batch process is particularlypreferred for steps b) and/or c).

1. The production of the foams in a batch process can be carried out inaccordance with examples 1 to 5. Preferably, this requires matching thescale of the pressure-proof equipment to the desired foam quantity andmaintaining the above-mentioned parameters such as pressure, temperatureand residence time of the polymer gel in the foaming agent atmosphere,for example. Possible pressure-proof equipment is preferably autoclaveswhich resist the required process parameters and are already used inindustrial processes. Hence, the polymer gels can preferably becontacted with the compressed foaming agent in a closed container andexpanded to a solid nanofoam after a sufficient thermostatting.

2. The production of the foams in a continuous process could be realisedin an extruder, for example. Since the polymer gels are easily formablealready at room temperature, they can be continuously processed in anextruder without supplying heat energy and foamed by adding the foamingagent by the process of the invention (NF-GAFFEL). In this process,energy is introduced into the system preferably in the form of shearingwhich provides for a steady increase of the polymer gel surface. Here,the formation of the homogeneous mixture of foaming agent andplasticiser in the polymer gel is preferably accelerated and thereforereduces the time of contact between foaming agent and polymer gel.Preferably, large-scale extruders allow adjusting the requiredparameters of pressure and temperature easily provided that the extruderis sufficiently gasproof. The time of contact between the foaming agentand the polymer gel can preferably be varied by the extrusion canallength and the screw speed. Preferably, the polymer gel filled withfoaming agent can be expanded at the extruder end to form a solidnanofoam by opening a valve. Preferably, a pressure gradient resultingin a slow expansion (t_(exp)≈20 s) is advantageous here (see Example2.4).

The invention will be further elucidated by the following,non-restrictive examples.

EXAMPLES Example 1: Polymethyl Methacrylate (PMMA) Nanofoams Example1.1: Production of a PMMA Nanofoam by the NF-GAFFEL Method

A PMMA gel was prepared by adding acetone to a sample of a conventionalacrylic glass and subsequently foamed with CO₂. The expansion timet_(exp) from the initial pressure p=250 bar to the normal pressure of 1bar was approx. 1 second. The mass ratio of PMMA to acetone in thepolymer gel was 1:3 (λ_(acetone)=0.75). The gel was contacted with CO₂in a high-pressure cell at p=250 bar, T=55° C. and t=15 min andsubsequently expanded. FIG. 1 shows the foam structure at two differentmagnifications.

The gel/foam composition was: σ=unknown, λ_(acetone)=0.75, n=unknown,ρ=unknown.

Example 1.2: Production of a PMMA Nanofoam by the NF-GAFFEL Method

A PMMA consisting of the monomer methyl methacrylate (MMA) and thecrosslinker N,N′-methylene bisacrylamide (MBAA) was polymerised usingthe radical starter azobisisobutyronitrile (AIBN) at T=95° C. and aperiod of 2 h. Example 1.2 illustrates the different impacts of twodifferent crosslinker concentrations on the obtained foam structure. Apolymer saturated with acetone was prepared and contacted with CO₂ andfoamed as in Example 1 at p=250 bar, T=55° C. and t=15 min. This exampledemonstrates the successful application of the NF-GAFFEL method using aPMMA sample with a precisely known composition.

The left-hand portion of FIG. 2 shows the effect on the resulting foamstructure at a crosslinker content of ν=0.2 mol % and the right-handportion of FIG. 2 shows a distinctly smaller foam structure due to theeffect of ν=0.7 of crosslinker.

The gel/foam composition was: σ_(AIBN)=0.004, λ_(acetone)=saturated,ν_(MBAA)=0.20 mol % (left)/0.70 mol % (right).

Example 1.3: Production of a PMMA Nanofoam by the NF-GAFFEL Method

A PMMA consisting of the monomer methyl methacrylate (MMA) and thecrosslinker N,N′-methylene bisacrylamide (MBAA) was polymerised usingthe radical starter azobisisobutyronitrile (AIBN) at T=95° C. and aperiod of 2 h as in Example 1.2. Example 1.3 illustrates the effect ofthe foaming temperature on the foam structure. For this purpose the PMMAgel was produced as in Example 2 and contacted with CO₂ and foamed atp=250 bar, t=15 min. The CO₂ contacting temperature and thus also theexpansion temperature was varied. The left-hand portion of FIG. 3 showsthe resulting foam structure at T=35° C., in the middle portion thetemperature was T=55° C. and in the right-hand portion the temperaturewas T=75° C.

The gel/foam composition was: σ_(AIBN)=0.004, λ_(acetone)=saturated,ν_(MBAA) 0.70 mol %.

Example 1.4: Production of a PMMA Nanofoam by the NF-GAFFEL Method

A PMMA consisting of the monomer methyl methacrylate (MMA) and thecrosslinker N,N′-methylene bisacrylamide (MBAA) was polymerised usingthe radical starter azobisisobutyronitrile (AIBN) at T=95° C. and aperiod of 2 h as in Examples 2 and 3.

Example 4 illustrates the effect of the residence time of the gels inthe CO₂ atmosphere on the foam structure. For this purpose the PMMA gelwas produced as in Example 2 and foamed at p=250 bar. The crosslinkerconcentration was ν=0.7 mol % and the foaming temperature was T=55° C.in all experiments.

FIG. 4 shows the results of three foaming experiments varying in thetime the gel was subjected to the CO₂ atmosphere. The left foam wasobtained after a saturation time of t=5 min, the middle foam after t=15min and the right after t=60 min.

The gel/foam composition was: σ_(AIBN)=0.004, λ_(acetone)=saturated,ν_(MBAA) 0.70 mol %.

Example 1.5: Production of a PMMA Nanofoam by the NF-GAFFEL Method

A PMMA consisting of the monomer methyl methacrylate (MMA), thecrosslinker N,N′-methylene bisacrylamide (MBAA) and the modifier2-ethylhexylthioglycolate (EHTG) was polymerised using the radicalstarter azobisisobutyronitrile (AIBN) at T=95° C. and a period of 2 h.Example 5 illustrates the effect of the modifier on the foam structureof two polymers at different crosslinker concentrations. For thispurpose the polymers were saturated with acetone and contacted with CO₂and foamed as in the above examples at p=250 bar, T=55° C. and t=15 min.The modifier concentration of p=0.50 mol % was the same in both foams.

The left-hand portion of FIG. 5 shows the effect on the resulting foamstructure at a crosslinker content of ν=0.7 mol % and the right-handportion of FIG. 5 shows the effect at ν=1.20 mol % of crosslinker. Themodifier (EHTG) has an enormous influence on the properties of the foampores. Monodisperse closed-cell foams having pore sizes in the lowermicrometre range (left) and foams with regions of different porediameters (right) can be produced easily. This allows to produce foamswith regions having different properties.

The gel/foam composition was: σ_(AIBN)=0.004, λ_(acetone)=saturated,ν_(MBAA)=0.70 mol % (left)/1.20 mol % (right), ρ_(EHTG)=0.50 mol %.

Example 2: Polystyrene (PS) Nanofoams Example 2.1: Production of PSNanofoams With Different Pore Sizes by the NF-GAFFEL Method

A PS consisting of the monomer styrene and the crosslinkerdivinylbenzene (DVB) was polymerised using the radical starterazobisisobutyronitrile (AIBN) at T=90° C. and a period of 2 h.

A PS gel was prepared from PS and the same weight of acetone(λ_(acetone)=0.50) and contacted with CO₂ in a high-pressure cell atρ=250 bar, T=55° C. and t=15 min and subsequently expanded. FIG. 6 showsthe foam structure of two samples containing different crosslinkerconcentrations. The left sample has a crosslinker concentration of ν=0.5mol % and the right sample has a crosslinker concentration of ν=1 mol %.

The gel/foam composition was: σ_(AIBN)=0.004, λ_(acetone)=saturated,ν_(DVB)=0.5 mol % (left)/1.00 mol % (right).

Example 2.2: Production of PS Nanofoams by the NF-GAFFEL Method

PS gels were produced as in Example 2.1 and contacted with CO₂ at p=250bar, T=55° C. and t=15 min and subsequently expanded. The crosslinkerconcentration (DVB) was ν=1 mol %. Three foaming experiments wereconducted at different temperatures. The foam in the left-hand portionof FIG. 7 was obtained at a temperature of T=35° C., the foam in themiddle portion at T=65° C. and foam in the right-hand portion at T=75°C.

The gel/foam composition was: σ_(AIBN)=0.004, λ_(acetone)=0.50, ν_(DVB)1.00 mol %.

Example 2.3: Production of PS Nanofoams by the NF-GAFFEL Method

PS gels were produced as in Example 2.1 and subsequently contacted withdifferent CO₂ pressures and foamed at T=65° C. after t=15 min. Thecrosslinker concentration (DVB) was ν=1 mol %. In FIG. 8 the left foamresulted from a CO₂ pressure of p=250 bar, whereas the right foam wasobtained at a CO₂ pressure of p=150 bar.

In both experiments the parameters are chosen such that they are abovethe binary miscibility gap between CO₂ and acetone.

The gel/foam composition was: σ_(AIBN)=0.004, λ_(acetone)=0.50, ν_(DVB)1.00 mol %.

Example 2.4: Production of PS Nanofoams by the NF-GAFFEL Method

PS gels were produced as in Example 2.1 and contacted with CO₂ at p=250bar, T=60° C. and t=15 min and subsequently expanded. The crosslinkerconcentration (DVB) was ν=2 mol %. The decisive difference to allprevious foaming experiments was the duration of the expansion processfrom 250 to 1 bar. FIG. 9 shows the foam obtained with a foaming time oft_(exp)≈20 s.

The gel/foam composition was: σ_(AIBN)=0.004, λ_(acetone)=0.50, ν_(DVB)2.00 mol %.

Example 3: Polyvinyl Chloride (PVC) Nanofoams; Production of a PVCNanofoam by the NF-GAFFEL Method

A saturated PVC gel was prepared by adding acetone to a sample of aconventional PVC polymer and subsequently foamed with CO₂. The gel wascontacted with CO₂ in a high-pressure cell at p=250 bar, T=70° C. andt=10 min and subsequently expanded. FIG. 10 shows the foam structure attwo different magnifications.

The gel/foam composition was: σ=unknown, λ_(acetone)=saturated,ν=unknown, ρ=unknown.

Example 4: Polyethylene (PE) Nanofoams; Production of a PE Nanofoam bythe NF-GAFFEL Method

A saturated PE gel was prepared by adding cyclohexane to a sample of aconventional PE polymer at 60° C. and subsequently foamed with CO₂. Thegel was contacted with CO₂ in a high-pressure cell at p=250 bar, T=70°C. and t=15 min and subsequently expanded. FIG. 11 shows the foamstructure at two different magnifications.

The gel/foam composition was: σ=unknown, λ_(cyclohexane)=saturated,ν=unknown, ρ=unknown.

Example 5: Preparation of 10 g of Nanoporous Polystyrene Particles

Ten grams of crosslinked polystyrene particles (mean diameter 1 mm, 1mol % of DVB as crosslinker) is contacted with 20 g of acetone in asealed vessel for 180 minutes at room temperature and under normalpressure. Due to the different refractive indexes of the polymer and thepolymer gel, swelling could be followed visually to determine when thepolystyrene particles were completely converted into the polymer gel.The polystyrene gel particles were dimensionally stable and had aspherical shape but were—contrary to the starting polymer—deformable byslight mechanical impact. Then, the swollen polystyrene particles weresubjected to a CO₂ atmosphere at 200 bar and 70° C. for 90 minutes. Apressure relief resulted in nanoporous polystyrene particles with a meandiameter of 2 mm and a density <0.30 g/cm³ and a mean nanopore diameter<500 nm (see FIG. 12).

Example 6: Determination of the Structure of the Nanoporous Materials

In order to image the structure of the produced nanoporous materials,first a fresh fracture edge was created. Subsequently, the sample wasfixed on the sample plate with the fracture edge facing upward. In orderto dissipate the charge generated during measurement, conductive silverlacquer was used for fixation. Prior to imaging, the sample was coatedwith gold in order to avoid local charging effects. For this purpose theK950X coating system with the K350 sputter attachment from EMITECH wasused. In all cases gold sputtering was performed under an argon pressureof approx. 10⁻² mbar, always applying a current of 30 mA for 30 seconds.The layer thickness of the gold layer coated in this way wasapproximately between 5 and 15 nm. The electron photomicrographs weretaken with a device of the SUPRA 40 VP type from ZEISS. Accelerationvoltages up to 30 kV and a maximum resolution of 1.3 nm are possiblewith this device. Micrographs were recorded with the InLens detector atan acceleration voltage of 5 kV. In order to determine the mean porediameter and the mean web thickness of the nano- and microporous foams,a micrograph taken with the described scanning electron microscope waschosen and at least 300 pores or webs were measured with the DatinfMeasure computer program to ensure a sufficiently good statistics. Sinceeach scanning electron micrograph contains only a limited number ofpores and webs, several scanning electron micrographs are used for thedetermination of the mean pore and web diameters. Here it should beensured that the magnification is chosen such that the error in thelength determination is kept as small as possible. An example is shownin the Annex in FIG. 13.

The features of the invention disclosed in the present description, inthe drawings as well as in the claims both individually and in anycombination may be essential to the realization of the variousembodiments of the invention.

What is claimed is:
 1. A micro- or nano-porous polymer material, whereinthe polymer material is of a density in a range from 10 to 300 kg/m³,wherein when counting at least 300 pores of the polymer material, anaverage size of the at least 300 pores is in a range from 0.05 to 0.5μm, wherein the micro- or nano-porous polymer material is not soluble inits own monomer, wherein the polymer material is selected from the groupconsisting of a polystyrene consisting of styrene as the monomer anddivinyl benzene (DVB) as a crosslinker and a polymethyl methacrylate(PMMA) consisting of methylmethacrylate as the monomer and N, N′methylene bisacrylamide (MBAA) as a crosslinker.
 2. The micro- ornano-porous polymer material according to claim 1, wherein the polymermaterial and/or the starting material are partially crosslinked.
 3. Themicro- or nano-porous polymer material according to claim 1, whereinarithmetic mean of thickness of webs between pores of the polymermaterial is in a range from 5 to 50 nm.
 4. The micro- or nano-porouspolymer material according to claim 3, wherein the arithmetic mean ofthickness of webs is in a range from 10 to 35 nm.
 5. The micro- ornano-porous polymer material according to claim 1, wherein the micro- ornano-porous polymer material is partially closed-cell.
 6. The micro- ornano-porous polymer material according to claim 1, wherein the polymermaterial has a density in a range from 30 to 200 kg/m³.
 7. The micro- ornano-porous polymer material according to claim 1, obtained by a methodcomprising: swelling a polymer starting material using a plasticizer ata predetermined temperature to make the polymer starting materialvisco-elastic deformable, wherein the polymer starting material is atleast partially crosslinked and the crosslinker content in the polymerstarting material is in a range from 0.01 to 10 mol %, wherein thepredetermined temperature is in a range from 0 to 100° C.; subsequently,contacting the swollen polymer starting material with a foaming agentunder a first pressure; and subsequently, depressurizing from the firstpressure to a second pressure such that the swollen polymer startingmaterial further expands and solidifies to obtain a micro- ornano-porous material.
 8. A molded body made of a micro- or nano-porouspolymer material according to claim
 1. 9. The molded body according toclaim 8, wherein the moulded molded body is sealed.
 10. The mouldedmolded body according to claim 8, wherein the molded body has a thermalconduction in a range from 1 to 30 mW/(m·K).
 11. The molded bodyaccording to claim 10, wherein the molded body has a thermal conductionin a range from 10 to 26 mW/(m·K).
 12. The micro- or nano-porous polymermaterial according to claim 1, wherein a content of the crosslinker is0.1 to 1 mol %.